The manufacture of plastics involves the precise blending of various chemicals under carefully controlled process conditions. Variations of these process steps can substantially affect the characteristics and performance of the plastic. As a result, samples from batches of plastic are routinely tested to assess the performance of the plastics from a particular batch.
One widely used and important test assesses the reaction of the plastic to specified impact forces. The plastics industry employs an impact test performed precisely in accordance with ASTM standard #256 Method A (Izod type). In particular, the test employs a rectangular plastic specimen having a length of 2.500 inches, a width of 0.500 inch and a predetermined thickness of 0.125 inch, 0.250 inch or 0.500 inch. A V-shaped notch is formed in one edge of the plastic at a location midway along the length of the specimen. The notch is cut to a specified depth of 0.100.+-.0.002, such that the width of the specimen below the notch is 0.400 inch.+-.0.002 inch. The specimen is then securely clamped in a vise such that the apex of the notch is precisely aligned with the top of the vise. The portion of the specimen extending above the top of the vise is then impacted by a specified weight gravitationally falling through a specified arc. The particular weight employed in these tests is selected in accordance with the anticipated characteristics of the plastics being tested. The impact of the weight on the specimen is measured in appropriate units (e.g. foot-pounds/inch), with the type of break being noted and recorded (e.g. no break, partial break, hinged break or complete break). The test results of both the break type and the corresponding impact are then analyzed to determine if the plastic performed to its specifications.
The above described impact test is often performed for each batch of plastic being made. ASTM standards recommend that the impact tests be performed on ten specimens, with average readings then being compared to specifications.
The prior art impact testing procedures are labor intensive, slow, costly and often yield inaccurate results. In particular, the first step in the prior art testing procedure is to manually check the dimensions of each specimen with micrometers. If the specimen falls outside the established dimensional tolerances, it is discarded. Acceptable specimens are then manually notched. The prior art notching apparatus is intended only to receive two specimens at a time, with the second of the two specimens being discarded in view of inaccuracies that are likely as the prior art notch apparatus completes its cut. Attempts to insert more than two specimens in the prior art notch apparatus results in an upward or downward bowing of the linear array of specimens, with corresponding inaccuracies in the dimensions of the notch. In this regard, it must be emphasized that inaccuracies in either the dimensions of the specimen or the dimensions of the notch significantly affect the response of the specimen to the impact, and thereby yield inaccurate test results. The prior art manual measurement of the specimen and manual notching make errors likely with correspondingly inaccurate test results.
After the specimen is notched by the prior art apparatus, each specimen is manually mounted into an appropriate vise for completion of the prior art impact test. The test technician visually observes the specimen during and immediately after the impact test and records the foot-pounds/inch reading of the prior art impact tester. In the typical prior art apparatus, these individual test readings are manually recorded for subsequent analysis. Recently, certain improvements in the recordings of impact test results have been made. In particular, an impact tester has been employed with control means which store and analyze the foot-pounds/inch data from a plurality of tests, thereby avoiding the need for manual recording and analysis. An example of such a recent prior art impact testing apparatus is the Monitor/Impact tester sold by Testing Machines Inc.
Despite the recent advances in plastic impact testing devices, the testing processes continue to be extremely labor intensive, slow, costly and subject to errors in the measurement, notching and visual observation portions of the tests. For example, it is estimated that the notching of the above identified specimens of 0.125 inch thickness takes approximately 7.5 seconds per specimen with the prior art notching machinery. The verification of the notch depth is estimated to take approximately ten seconds per sample, while the verification of the thickness and other dimensional measurements of the specimen is estimated to take approximately five seconds per specimen. The actual impact testing of each specimen is estimated to take approximately 22 seconds per specimen, while the recording of the results and the calculations of the foot-pounds/inch may take an additional 15 seconds per specimen. This recording and calculating step can be substantially eliminated with the recently developed Monitor/Impact tester of Testing Machines Inc. However, even these recently developed impact test devices do not improve the time or quality of the notching, notch verification and thickness measurement steps as identified above. In view of these estimated times, an average test run consisting of 50 specimens would take approximately 60 minutes of operator using the typical prior art equipment, and would take approximately 45 minutes of operator time using a system that automatically records test results such as the Monitor/Impact tester of Testing Machines Inc.
In view of the substantial times involved, many plastics companies have full-time staff dedicated to the preparation of specimens and actual testing. The characteristics of the prior art systems require the staff members to devote their full attention to the tasks of preparing specimens and running impact tests.
In view of the above, it is an object of the subject invention to provide an efficient impact testing apparatus and process.
It is a further object of the subject invention to provide an impact testing apparatus and process that substantially minimizes the labor costs associated with plastics impact testing.
Another object of the subject invention is to provide an impact testing apparatus that enables more accurate impact testing results.
A further object of the subject invention is to provide an impact testing apparatus that automatically assesses and identifies the type of break in the plastic specimen.